Anti-accretion additives for drilling fluids

ABSTRACT

Additives for drilling fluids, in particular for water-based drilling fluids are described which when added to the fluid at levels of up to 10% weight by volume reduces the accretion and bit-balling tendencies of shale cuttings exposed to said fluids. The additives are based on phosphonate chemistry, and are of the general class (I), wherein R, R′ and R″ are radicals exclusively containing H atoms or combinations of H, C, O or P atoms up to a maximum of 100 atoms.

This application is a 371 of PCT/GB99/00298 filed Jan. 28, 1999, whichclaims priority from GB 9803249.3 filed Feb. 17, 1998.

This invention relates to anti-accretion additives for drilling muds.

BACKGROUND OF THE INVENTION

Bit-balling and cuttings accretion are problems encountered whendrilling shales, particularly with water-based muds. Shale cuttings canadhere to each other and to the bottom hole assembly and cuttingsurfaces of the bit. Gradually a large plastic mass builds up which canblock mud circulation and reduce rates of penetration. There is a“danger zone” of clay plasticity for balling and accretion, related tothe water content of the clay or shale, which can be defined in terms ofthe Atterberg limits of soil mechanics. In the dry zone the clay has toolittle water to stick together and it is a friable and brittle solid. Inthe wet zone the material is essentially liquid like with very littleinherent strength and can be washed away.—Intermediate to these zones,i.e., in the danger zone, the shale is a sticky plastic solid withgreatly increased agglomeration properties and inherent strength.

When cuttings are exposed to conventional water-based muds they usuallyimbibe water and pass rapidly through these different zones, eventuallydispersing. However recent advances in drilling fluid technology havedeveloped highly inhibitive muds which appear to reduce the hydration ofshale and in doing so maintain the cuttings in the danger or plasticzone contributing to increased accretion and bit-balling. Fieldexperiences with glycol, phosphate and silicate muds in particular haveshown accretion problems.

U.S. Pat. No. 5,639,715 describes additives for bit-balling preventionbased on sulphonosuccinate chemistry.

Phosphorus based additives and compound have been used in the oilfieldindustry mainly for the purpose of enhancing oil recovery fromproduction wells.

It is the object of the present invention to find alternatives to theknown methods of preventing accretion.

SUMMARY OF THE INVENTION

The invention is an additive for drilling mud. The additive reduces theaccretion and bit-balling tendencies of cuttings exposed to said muds.The additives are based on phosphonate chemistry, and are preferably ofthe general class:

wherein R, R′ and R″ are radicals exclusively containing H atoms orcombinations of H, C, O or P atoms up to a maximum of 100 atoms.

In a more preferred embodiment, the additives are based on the formula

wherein R₁, R₂ and R₃ are radicals exclusively containing H atoms orcombinations of H, C, O or P atoms up to a maximum of 100 atoms.

In a preferred embodiment of the invention, the additives are containingnot more than one phosphor atom.

In another preferred embodiment of the invention, the additive is aphosphor derivative of the succinic acid or short chain phosphorylatedhydrocarbons.

Additives according to the invention are added to the drilling fluid atlevels 0.1-10%, preferably 1-5%, weight by volume (% kg/liter). Thedrilling fluid itself may be oil based, though it is recognized thataccretion tends to be less pronounced in drilling muds of this kind.Therefore, the preferred drilling fluid in accordance with the presentinvention is water based, even more preferably a reactive anionic baseddrilling fluid, such as silicate or phosphate based muds. Furtheradditives as known in the art may be added to impart other desiredproperties to the mud system. Such known additives include viscosifyingagents, filtrate reducing agenst, and weight adjusting agents. Otherpreferred additives are shale-swelling inhibitors, such as saltsglycol-, silicate- or phosphate-based agents, or any combinationthereof.

These and other features of the invention, preferred embodiments andvariants thereof, and further advantages of the invention will becomeappreciated and understood by those skilled in the art from the detaileddescription below.

MODE(S) FOR CARRYING OUT THE INVENTION

A test used to determine the anti-accretion properties of additivesinvolves squeezing shale or clay cuttings between two steel plates witha given force causing them to stick to each other and the plates. Theforce required to slide the plates apart is then determined using aforce gauge or spring balance.

Oxford clay cuttings of size 2-4 mm were soaked in the test fluid for 15minutes. The excess mud was drained from the cuttings using a sieve (500micron mesh). A small pile of cuttings (5-10 g) was put onto the baseplate of the tester. The pile was roughly levelled and the top platereplaced over the cuttings. A PTFE spacer was placed on top of the topplate. A screw-mounted plunger in the tester housing was wound downuntil it made contact with the spacer. A torque wrench was used totighten the plunger onto the top plate. The standard torque was 75inch-pounds (˜9N.m). Immediately on reaching this value, the plunger waswound back sufficiently to remove the spacer. A force gauge or springbalance was then connected to the top plate. The tension on the topplate was then increased by pulling on the force gauge until the platebreaks free from the cuttings bed. The maximum force recorded was thefreeing force for the plate or accretion value. Values can range from1.0 to above 20.0 kg force.

The phosphonate based additives tested in accordance with the aboveprocedure are added to a water-based mud containing tetrapotassiumpyrophosphate (TKPP) and consisting of

1000 ml fresh water (base)

85.5 g tetrapotassium pyrophosphate (shale inhibitor)

2.85 g xanthan gum (viscosifier)

11.4 g carboxy methyl cellulose of low viscosity grade (filtratereducer)

42.75 g simulated drill solids

barite (weighting agent) to density 1.08 sg.

NaOH to pH 9.2

biocide

Baseline accretion values were established as:

Simple polymer mud   5 kg TKPP mud 21.7 kg

The anti-accretion additives were then added to the TKPP mud at levelsof 1-5%.

Additives which reduced the accretion value from >10 kg to 9 kg or belowwere:

Hydrolysed polymaleic acid

3-phosphonopropionic acid

succinic acid

propyl phosphonic acid

dibutyl-butyl phosphonate

hydroxyphosphonoacetic acid

dimethylpropyl phosphonate

phosphorous acid

diethyl-ethylphosphonate

ethylmethacylate phosphate

tri-ethyl phosphonoacetate

tetramethyl phosphonosuccinate

phosphonosuccinic acid

2-hydroxyethyl phosphonic acid.

The last five additives (Additives 9-14) were the found most effective.For those the following values were recorded:

TKPP mud + (%) additive: Accretion value 1% diethyl-ethylphosphonate 8kg 5% diethyl-ethylphosphonate 7 kg 5% ethylmethacrylate phosphate 6 kg1% tri-ethyl phosphonoacetate 8 kg 5% tri-ethyl phosphonoacetate 5 kg 5%tetramethyl phosphonosuccinate 7 kg 5% phosphonosuccinic acid 7 kg 5%2-hydroxyethyl phosphonic acid. 7 kg

In a second series of tests with the additives, silicate mud of thefollowing composition was used:

1000 ml sea water (base)

131 g Na silicate, a solution of 14% NaOH and 27%SiO_(2 (shale inhibitor))

117.5 g KCl (shale inhibitor, weighting agent)

20 g Polyanionic cellulose (filtrate reducer)

5 g Xanthan gum (viscosifier)

NaOH to adjust pH to 12.

Baseline accretion values were established as:

simple polymer mud  9.5 kg silicate mud 17.7 kg

The anti-accretion additives were tested in the silicate mud at 1%(w/v):

Silicate mud + (1%) additive: Accretion value diethyl-ethylphosphonate11.1 kg tri-ethyl phosphonoacetate 11.35 kg  tetramethylphosphonosuccinate 9.96 kg phosphonosuccinic acid 10.8 kg 2-hydroxyethylphosphonic acid 11.4 kg

In most cases the accretion value has been reduced significantly, downto the levels of a simple polymer mud.

What is claimed is:
 1. A drilling fluid comprising water as basecomponent; a viscosifying agent to increase the viscosity of the fluid;a filtrate reducing agent; a weighting agent to adjust the density ofthe fluid; a shale swelling inhibition agent comprising phosphate orsilicate based compounds; and an additive for a drilling fluid,consisting of a compound in accordance with the formula

 wherein R, R′ and R″ are radicals exclusively containing H atoms orcombinations of H, C, O or P atoms up to a maximum of 100 atoms.
 2. Thedrilling fluid of claim 1, wherein R, R′ and R″ are radicals exclusivelycontaining H atoms or combinations of H, C or O.
 3. The drilling fluidof claim 1, wherein the additive consists of a compound in accordancewith the formula

wherein R₁, R₂ and R₃ are radicals exclusively containing H atoms orcombinations of H, C, O or P atoms up to a maximum of 100 atoms.
 4. Thedrilling fluid of claim 3, wherein R₁, R₂ and R₃ are radicalsexclusively containing H atoms or combinations of H, C or O.
 5. A methodof preventing accretion of cuttings in a borehole, said methodcomprising the step of preparing a drilling fluid comprising aviscosifying agent to increase the viscosity of the fluid, a filtratereducing agent, a weighting agent to adjust the density of the fluid, ashale swelling inhibition agent comprising phosphate or silicate basedcompounds and an additive for a drilling fluid, consisting of a compoundin accordance with the formula

wherein R, R′ and R″ are radicals exclusively containing H atoms orcombinations of H, C, O or P atoms up to a maximum of 100 atoms.
 6. Themethod of claim 5, wherein the additive is added in a concentration ofup to about 10% weight by volume of the drilling fluid.
 7. The drillingfluid of claim 1, wherein the additive is based on a phosphor derivativeof the succinic acid.
 8. The drilling fluid of claim 1, wherein theadditive is based on a short phosphorylated hydrocarbon.
 9. The drillingfluid of claim 1, comprising the additive in a concentration of up toabout 10% weight by volume.
 10. The drilling fluid of claim 1, whereinthe shale swelling inhibition agent comprises phosphate based compounds.11. The drilling fluid of claim 1, wherein the shale swelling inhibitionagent comprises silicate based compounds.